Many devices have been disclosed for measuring angular displacement. Typical devices often utilize detection of shifts in fringe patterns of diffracted light.
U.S. Pat. No. 5,355,220 to Kobayashi et al. discloses a light from a source radiated onto a diffraction grating to generate diffracted lights of different orders of diffraction permitting detection of light and dark stripes. Movement is measured by direct detection of movement of the stripes.
U.S. Pat. No. 5,486,923 to Mitchell et al. discloses a grating which concentrates light having a pre-selected wavelength into + and − first orders while minimizing the zero order. The diffracted orders of light illuminate a polyphase detector plate.
U.S. Pat. No. 5,559,600 to Mitchell et al. discloses a grating concentrating a pre-selected wavelength into positive and negative first orders. A polyphase periodic detector has its sensing plane spaced from the scale at a location where each detector element responds to the positive and negative first orders without requiring redirection of the diffracted light.
U.S. Pat. No. 5,909,283 to Eselun uses a point source of light directing a beam at an angle onto a movable scale. Diffraction beams are generated which are intersected by an optical component such as a Ronchi grating so as to form Moire fringe bands. An array of sets of photodetectors are positioned to intercept the bands of the Moire pattern and emit signals that are electronically processed to indicate displacement of the scale.
U.S. Pat. No. 7,002,137 to Thorburn discloses an optical encoder including a scale, the scale including an optical grating and an optical element; a sensor head, the sensor head including a light source and a detector array both of which are disposed on a substrate, the scale being disposed opposite the sensor head and being disposed for movement relative to the sensor head. A distance between the scale and a Talbot imaging plane closest to the scale being equal to d. The sensor head being disposed within a region bounded by a first plane and a second plane, the first plane being separated from the scale by a distance substantially equal to n times d plus d times x, the second plane being separated from the scale by a distance substantially equal to n times d minus d times x, n being an integer and x being less than or equal to one half. The light source emits a diverging beam of light, the diverging beam of light being directed towards the scale, light from the diverging beam of light being diffracted by the grating towards the detector array. A mask is disposed between the scale and the sensor head, the mask defining an aperture, the mask remaining substantially fixed relative to the sensor head, the aperture being sized and positioned to substantially prevent fifth order beams diffracted from the grating from reaching the detector array.
However, high accuracy is often a requirement for these devices, for example, micron range accuracy is often required. Many conventional devices have problems that limit their accuracy, reliability, calibration, and ease of mass production, and they are generally expensive when such fine resolution is required. The expense is due in part to the requirement to assemble these parts in precise locations relative to one another. Therefore, there is a need for highly accurate and reliable optical encoders that can be mass produced in an inexpensive manner.